How to manipulate Gradient Shader
The Gradient texture outputs a numerical value between 0 and 1.  You can create a color gradient from this by plugging the gradient texture into a Color Ramp node.



The Color Ramp takes a value between 0 and 1 (values outside this range are clamped) and outputs the color from the proper place along the set gradient.  A value of 0 will output the color on the far left, a value of 1 will output the color on the far right.

You can create a custom gradient by adding color swatches, Cycles interpolates the colors between each swatch.  Add a new color swatch by clicking the "+" button, select a swatch to change its color and position.



As a technical side note, the gradient node is actually a texture not a shader.  A shader is a set of instructions that tell Cycles how light interacts with the surface or volume of an object, a texture is an array of colors.